There exist two general classes of seismometers known as short-period and broadband seismometers. Broadband seismometers can measure weak ground motion at low frequencies, whereas short-period seismometers cannot. The amplitude of seismic ground motion tends to drop rapidly with a frequency below 1 Hz, while the self-noise level of seismometers tends to rise, so that it is much easier to achieve a good signal-to-noise ratio at frequencies above 1 Hz (“short periods”) than at lower frequencies.
Most broadband and some short-period seismometers are of the force-feedback type. All force-feedback seismometers comprise an electronic means for applying a force to hold a movable inertial mass at a predetermined null position with respect to a frame resting on the ground. In short-period seismometers with electronic force feedback, the feedback is strong enough to balance effects of tilt and temperature on the seismometer mechanics as well as inertial forces due to seismic ground motion; however this strong electronic feedback causes noise that makes the seismometer unable to measure weak ground motion at low frequencies. In broadband seismometers the electronic force feedback is made weaker (by design) to improve noise performance, but an additional mass centering adjustment mechanism is required for adjusting the internal mechanics of the seismometer to balance the effects of tilt and temperature. This mass centering adjustment mechanism adds to the size and cost of the seismometer and impairs reliability. Furthermore it is inconvenient to adjust the internal mechanics each time the seismometer is installed at a new site and to re-adjust from time to time as the tilt or temperature of the installation changes.
Therefore there is a need for a broadband force-feedback seismometer that can accommodate a useful operating range of tilt and temperature in field service without adjustment of the internal mechanics. Also any design improvement that can reduce the size and cost of a broadband seismometer is desirable.